Field of the Invention
The invention relates to a method and a device for monitoring or limiting a power rise during startup of a nuclear reactor, in which a local power is detected in so-called xe2x80x9cpower region channelsxe2x80x9d in individual regions of the core, such as is described, for example, in International Publication No. WO 96/21929, corresponding to U.S. Pat. Nos. 5,875,221 and 5,978,429, and European Patent Application 0 496 551 A1, in normal operation through the use of measuring signals.
During startup of a nuclear reactor, the neutron flux level, which is proportional to the power of the reactor generated by nuclear fissions, is raised from a neutron flux source level at approximately 10xe2x88x929 of a rated power initially up to reaching a heating-up power which is, for example, approximately 10xe2x88x923 of the rated power. In that intermediate range of the reactor power, the rise in the neutron flux density virtually does not change the thermal conditions in the reactor core, that is to say, in particular, the temperatures of the nuclear fuel. As a result almost no reactivity feedback effects are produced which could influence the rate of rise of the neutron flux density. It is only above that intermediate range, that is to say in the percentage range of reactor power (lowermost power range) that, because of the then sensible heating of the nuclear fuel and because of the energy contribution to the coolant, which results therefrom (and is delayed by the fuel time constant), that a reactivity feedback corresponding to the reactivity coefficients of the reactor core begins. That reactivity feedback generally effects a continuous retardation of the power rise until it comes completely to a standstill.
The rise in the neutron flux density through the many decades of the intermediate range is effected by setting a slightly supercritical nuclear state. An effective multiplication factor of the core configuration keff is therefore raised slightly above 1. That purpose is served by suitable reactivity control elements which, in the case of the boiling water reactor type preferably considered herein, are generally control rods, having an absorption effect for neutrons which is specifically reduced by metered withdrawal from the core. The neutron flux density rises in accordance with an exponential function after a supercritical nuclear state is reached. The rate of rise can be characterized by specifying a so-called reactor period. The reactor period is that period of time in which the neutron flux density in the core changes by a factor e=2.718 . . .
The neutrons released by a nuclear fission are partly xe2x80x9cpromptxe2x80x9d neutrons, which are released immediately by the split nucleus, and partly xe2x80x9cdelayed neutronsxe2x80x9d, which originate from unstable follower nuclei.
In the case of a normal startup operation, the excess reactivity of the core (that is to say the part of the effective multiplication factor keff exceeding the value 1) is set in such a way that the delayed neutrons retain the determining influence on the rate of rise of the neutron flux density. In order to ensure effective controllability, it is customary to undertake the startup in such a way that the reactor period is more than 30 seconds.
However, because of operating errors or accidents, the excess reactivity during startup of the nuclear reactor can also become so large that the rate of rise of the neutron flux density is exclusively determined by the prompt neutrons with their very fast neutron cycle, and the delayed neutrons lose any influence on the rate of rise. That reactor state is denoted as xe2x80x9cprompt criticalxe2x80x9d. The associated reactor period is far below 1 second. The startup operation then changes into an xe2x80x9cexcursionxe2x80x9d, in the case of which the rated value of the reactor power is exceeded briefly, depending on the excess reactivity, before the power is caught by inherent reactivity feedback. Other than in the case of the below-prompt critical startup operations, in the case of excursions the power rise is not already caught in the lowermost power range.
In the case of the occurrence of an excursion, the task is set for the nuclear instrumentation of the nuclear reactor of automatically actuating an emergency reactor shutdown reliably and in good time in such a way that at least conceivable consequent excursions are suppressed.
A boiling water reactor, a conventional instrumentation system for monitoring and controlling in accordance with the prior art and a typical measuring range are considered below and explained in detail with the aid of FIGS. 1 and 2 as a preferred example.
In order to also take account of the most dangerous extreme case (xe2x80x9cconservativexe2x80x9d view), it may be assumed that in terms of order of magnitude the neutron flux level has already risen in that time in the range of the rated power (xe2x80x9cprimaryxe2x80x9d excursion). The fuel heating connected therewith has then effected an inherent decrease in the excess reactivity through the promptly acting fuel temperature reactivity coefficient. As a result the reactor power already drops again when the control rods actually begin to be inserted into the core. Thus, viewed conservatively, the emergency reactor shutdown can neither prevent the occurrence of the primary excursion nor decisively dampen its course. However, its aim is to suppress conceivable consequent excursions which could come about due to further extraction of the control rods causing the accident, and through dissipation of the thermal energy determining the reactivity negative feedback.
It is accordingly an object of the invention to provide a different method and device for monitoring the power rise during startup of a nuclear reactor provided with power range channels (diversitary excursion monitoring), which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, which monitor consequent excursions in order to further increase reliability with which a suitable countermeasure is initiated and which create an additional initiation of the countermeasure through a monitoring system that is diversitary, that is to say it operates according to a different method and is completely independent of an intermediate range UD system.
Evidently, a power range LD system, which likewise has a redundant layout, does not come into consideration in principle as such a monitoring device which is independent of the UD system and preferably diversitary. That is because the overload limit mark of the LD system has not so far fulfilled the requirements to be placed on the monitoring of reactivity excursions. Specifically, because of the very steep distributions of the neutron flux density in the core, which is typical of such accidents, the LD system detectors situated in the vicinity of the excursion center, which supply the main signal contribution for the LD channels, become saturated and cannot feed the full level of the signal substantially exceeding their measuring range into the LD channel. The otherwise very good track fidelity of LD channels is thus naturally no longer obtained in the case of saturation of assigned LVD signals, and the LD signal does not reach the overload limit mark, although the reactor power is actually much higher. Moreover, the overload limit mark is likewise unable to counteract weaker excursions, which cannot even reach that power.
The invention assumes, however, that the LD system can be used when use is made of a limit value which can be reliably monitored even in the case of an excursion. Thus, use is made for the LD system of the nuclear reactor of a limit value criterion which responds reliably even in the case of an excursion and is diversitary in relation to the RESA triggering derived from the UD system or WD system. The limit at which a countermeasure must be initiated is set within the measuring range of the LD channels at a power which is so low that saturation of the detectors of the LD system either does not yet generally occur there, or in any case can only have a negligible influence. Therefore, according to the invention, that limit value is disposed generally far below the nominal value of the reactor power, and therefore responds even to weaker excursions. In order to detect the excursion, the power band situated below that limit value is used as a filter which responds only to the rate of rise and therefore permits a proper, slow traversal of the power band without activating the countermeasure.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for monitoring a power rise during startup of a nuclear reactor, which comprises detecting power of a nuclear reactor in power range channels with measuring signals during normal operation; and monitoring at least the measuring signal of one power range channel to initiate a countermeasure when a power band in a lower part of a measuring range of the power range channel is traversed more quickly than a prescribed minimum time.
With the objects of the invention in view, there is also provided a method for longer term limitation of a power rise during startup of a nuclear reactor, which comprises detecting power of a nuclear reactor in power range channels with measuring signals during normal operation; monitoring at least the measuring signal of one power range channel for the reactor power; and continuing startup of the nuclear reactor only when the measuring signal for the reactor power does not exceed an upper limit of a power band after last exceeding a lower limit mark of the power band in a lower part of a measuring range of the power range channel for a duration of a prescribed minimum time.
The invention therefore provides, in the case of a nuclear reactor having a power which is detected in normal operation through the use of measuring signals from power range detectors in power range channels, for the power rise to be monitored, even in the case of startup, by already using at least the measuring signal of one power range channel. In this case, the fact that this countermeasure is not held to be necessary until a power band in the lower part of the measuring range of this power range channel is traversed more quickly than in a prescribed minimum time, is used as an actuating criterion for an emergency reactor shutdown (RESA) or a similar countermeasure.
As has already been explained, a primary excursion proceeds so quickly that it cannot be prevented. This is not necessary if the primary excursion is restabilized immediately without initiating consequent excursions which could lead to a longer term power rise. The invention therefore likewise provides for monitoring the measuring signal of a power range channel in order to prevent such consequent excursions, and thus for the purpose of longer term limitation of the power rise during startup of such a nuclear reactor. The startup of the nuclear reactor is continued only if this measuring signal for the reactor power after the last overshooting of a lower limit mark of a power band situated in the lower part of the measuring range of this power range channel does not exceed the upper limit of the power band during the duration of a prescribed minimum time.
In accordance with another mode of the invention, there is provided a method which further comprises forming the measuring signal of each power range channel for the reactor power by summing measuring signals of power distribution detectors distributed over a volume of a reactor core.
In accordance with a further mode of the invention, there is provided a method which further comprises fixing at least one of an operationally independent lower and upper limit mark for the power band.
In accordance with an added mode of the invention, there is provided a method which further comprises placing the power band in a lower third, preferably a lower quarter, of a rated power of the reactor.
In accordance with an additional mode of the invention, there is provided a method which further comprises prescribing at least one of a width of the power band and a minimum time in an operationally independent manner.
In accordance with yet another mode of the invention, there is provided a method which further comprises setting a width of the power band at less than ⅓, preferably less than ⅕, of a rated power.
In accordance with yet a further mode of the invention, there is provided a method which further comprises setting the minimum time at less than one minute.
In accordance with yet an added mode of the invention, there is provided a method which further comprises continuously detecting the reactor power with measuring signals of a plurality of power range channels, and redundantly monitoring the reactor power.
In accordance with yet an additional mode of the invention, there is provided a method which further comprises monitoring signals of additional neutron flux detectors during startup of the reactor having a reactor core from which control rods are being withdrawn. In accordance with again another mode of the invention, there is provided a method which further comprises monitoring the power of the reactor core for maintenance of current maximum values with the aid of the additional neutron flux detectors, and varying the current maximum value as a function of operation during startup.
With the objects of the invention in view, there is also provided a device for monitoring the startup operation of such a core, which includes a logic circuit connected to at least one power range channel that is activated for a prescribed minimum time when a lower limit mark for a power band situated in the lower part of the measuring range of the power range channel is exceeded. This logic circuit then sets a signal for initiating countermeasures whenever the measuring signal of the power range channel for the reactor power exceeds an upper limit of the power band.
In accordance with another feature of the invention, there is provided an evaluation circuit, the logic circuit being one of a plurality of redundantly operating logic circuits connected to the evaluation circuit and to the power range channels.
In accordance with a further feature of the invention, there is provided an actuator for inserting control rods into a reactor core and for shutting down the reactor, the actuator to be excited by the signal for initiating countermeasures.
In accordance with a concomitant feature of the invention, there are provided additional neutron flux detectors, and a monitoring device connected to the additional neutron flux detectors, the actuator to be additionally activated by the monitoring device.
The use of time windows in the monitoring of the reactor power and of the measuring signals of power range detectors has certainly already been proposed in connection with oscillations which can be caused by hydrodynamic instabilities and likewise constitute a malfunction. However, such time windows serve the purpose of activating further time windows during monitoring of the same measuring signal, or are activated by a different activation signal derived from the measuring signal. The present invention has nothing to do with such oscillations. Instead, the present method and the present device are preferably already active without such a preceding activation signal derived from the measuring signal, and are also not used to activate further instances of monitoring or time steps used for monitoring the measuring signal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for monitoring the power rise during startup of a nuclear reactor (diversitary excursion monitoring), it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.